Add initial set of utility plugins for the template system:
- datetime: Date/time formatting and manipulation
- fetch: HTTP content retrieval and processing
- file: File system operations and content handling
- sys: System information and environment access
- text: String manipulation and formatting operations
Each plugin includes:
- Implementation with comprehensive test coverage
- Markdown documentation of capabilities
- Integration with template package
This builds on the template system to provide practical utility functions
while maintaining a focused scope for the initial plugin release.
